Optical waveguide device

Abstract: In order not to vary a light dividing ratio, a light insertion loss and a light extinction ratio, even if an environmental temperature is varied, an optical waveguide device (51) has at least one optical waveguide substrate made of ferroelectric crystals, an optical waveguide (54) formed in the optical waveguide substrate, a first crystal plane (51b) formed in the optical waveguide substrate in which charges of one polarity are generated due to pyroelectrical effects, a second crystal plane (51c) made of ferroelectrical crystals in which charges of the other polarity are generated, a first conductive layer (56a) and a second conductive layer (56c) wherein the first conductive layer is conducted electrically to the first conductive layer.

Description: The present invention relates to optical waveguide devices comprising an optical waveguide substrate made of ferroelectric crystals. In the fields of optical measurement systems, optical communication systems, etc., it is known to manufacture optical waveguide devices by forming three dimensional optical waveguides in various electro-optics crystals. As for the optical waveguide devices, optical branch devices, optical modulation devices, optical polarization devices, optical switches, multiplexer, and so on are known. The optical waveguide devices have such advantages as small size, good stability, low necessary power and high speed signal transmission. The inventors investigated the use of the optical waveguide device, which is manufactured by forming the three-dimensional optical waveguide of Y-branch type in the substrate made of LiNbO 3 , as a modulator for fiber optic gyroscopes. In this modulator, a light impinging upon one end of the optical waveguide is divided by a ratio of 50:50% at a branch portion of the optical waveguide. One of the divided light is phase-modulated by applying a predetermined voltage. Phase differences due to Sagnac effects, which are generated by a rotation of the objected optical systems, are detected by the optical waveguide device. The thus obtained phase difference is introduced into a formula showing the relation between a rotation angular rate of the optical system and a phase difference to obtain a rotation angular rate. The inventors further investigated an intensity modulator of Mach-Zehnder type formed by arranging an optical waveguide of Mach-Zehnder type in the substrate. In this intensity modulator, a light impinging upon the intensity modulator is divided. Respective divided lights are phase-modulated by applying a voltage thereto. Then, the modulated lights are interfered to vary the intensity. During these investigations, the inventors faced to the problems mentioned below. In the modulator for fiber optic gyroscope, it is necessary to set the dividing ratio substantially equal to 50:50. However, especially in the modulator for fiber optic gyroscope, an admission range of the dividing ratio is very narrow. This is because, in the case that the dividing ratio is out of 50:50, the detected rotation angular rate is largely varied even if the rotation rate of the optical system is not varied. For example, if it is assumed that the dividing ratio is temporarily varied from 50:50 to 40:60, the obtained rotation angular rate is varied about 20%. Therefore, in the modulator for fiber optic gyroscope, it is required to set the dividing ratio within 48:52∼50:50. Moreover, the optical waveguide device is principally required to act stably along a wide temperature range. However, if an environmental temperature of the optical waveguide device was varied, the dividing ratio at the branch portion of the optical waveguide was temporarily varied largely. As a result, the dividing ratio of 50:50 was varied to 30:70, especially to substantially 100:0. Moreover, in addition to the light dividing ratio variation, a light insertion loss of the optical waveguide device occurred. Further, in the Mach-Zehnder intensity modulator, if the light dividing ratio was varied, an extinction ratio is varied largely, and the light insertion loss of the overall optical waveguide device is temporarily increased at the same time. JP-60-257325 discloses an optical waveguide device including branch arms diverging from a junction point, and extending to a modulating region. A rectangular guard electrode surrounds the branch point and modulating region. The guard electrode is said to reduce temperature dependence and a pyroelectric effect. It is an object of the invention to provide an optical waveguide device comprising an optical waveguide substrate made of ferroelectric crystals, and an optical waveguide formed in a principal plane of the optical waveguide substrate, at least one functional portion for a light dividing or a light coupling being formed in the light waveguide, in which, a light insertion loss, a light dividing ratio and an extinction ratio are not varied, even if an environmental temperature of the optical waveguide device is varied. The invention provides an optical waveguide device according to claim 1. In the construction mentioned above, the inventors investigated the reason for varying the light insertion loss, the light dividing ratio and the extinction ratio at the functional portion of the light waveguide. As a result, we found that these reasons were due to pyroelectrical effects generated by a variation of the environmental temperature. That is to say, the pyroelectrical effects function to generate a large number of charges in a crystal plane of the optical waveguide substrate in a spontaneous electric polarization direction, and electric fields due to the charges were largely interfered to a light transmission at the functional portions. In the invention, the first conductive layer was formed in one region defined from a boundary between the first crystal plane and the major plane to the functional portion, and the second conductive layer was formed in the other region defined from a boundary between the second crystal plane and the major plane to the functional portion. Then, the first conductive layer and the second conductive layer were electrically conducted. As a result, we found that variations of the light insertion loss, the light dividing ratio and the extinction ratio were extraordinarily reduced. The reason for reducing the variations mentioned above is not obvious. However, in this case, the conductive layers are arranged in the regions defined from the functional portions of the optical waveguide to respective crystal planes in which the charges are generated due to the pyroelectrical effects. Therefore, if the charges are generated in respective crystal planes, lines of electric force become coarse near the conductive layers, and thus a potential gradient between the conductive layers becomes small accordingly. Therefore, we think that the pyroelectrical effects to the functional portions become small. Moreover, since the first conductive layer and the second conductive layer are electrically conducted, potentials of respective conductive layers arranged to both side portions of the functional portion become almost equal, and thus the pyroelectrical effects can be reduced. Since the inner edges of respective conducting layers are disposed along side the branch arms of the optical waveguide, the space between the respective conducting layers and the optical waveguide, and the space between the major plane and an edge portion, can be small. No conductive layers are preferably formed to both of the first crystal plane and the second crystal plane in which the charges are directly generated, and the charges instantaneously generated in the crystal planes are not neutralized. However, even if the charges generated in the crystal planes are not directly neutralized, it is possible to prevent the affections instantaneously applied to the functional portions at which a light is divided. As for the pyroelectrical effects, in the Japanese Patent Laid-open Publication No. 5-88125, a buffer layer is formed on a surface of an optical waveguide, and an antistatic layer is formed on the buffer layer. Moreover, a control electrode is formed on the antistatic layer. However, in this invention, the charges generated by the pyroelectrical effects are deviated ununiformly in the crystal surface due to an effect of the control electrode. The antistatic layer only prevents the charge ununiformity, and does not function to neutralize instantaneously the charges generated by the pyroelectrical effects. Further, the optical waveguide has a linear shape, and thus there is no problem about a variation of the light dividing ratio. Moreover, in the Japanese Patent Laid-open Publication No. 2-257108, a pair of ferroelectric substrates are connected via a conductive member in such a manner that respective spontaneous electric polarization are reversed with each other so as to reduce the pyroelectrical effects. However, in this invention, the charges generated in connection surfaces of the ferro-electric substrates can be neutralized, but the charges generated in the other surfaces than the connection surfaces cannot be neutralized. Therefore, electric fields remain in the ferroelectric substrates, and thus the pyroelectrical effects cannot be eliminated. Further, the optical waveguide has a linear shape, and thus there is no problem about a variation of the light dividing ratio. Exemplary embodiments of the invention will now be described with reference to the following figures. Fig. 1 is a plan view depicting one embodiment of an optical waveguide device according to the invention; Fig. 2 is a plan view showing another embodiment of the optical waveguide device according to the invention; Fig. 3 is a plan view illustrating another embodiment of the optical waveguide device according to the invention; Fig. 4 is a plan view depicting another embodiment of the optical waveguide device according to the invention; Fig. 5 is a plan view showing another embodiment of the optical waveguide device according to the invention; Figs. 6a and 6b are plan views respectively depicting one embodiment of a conventional optical waveguide device; Fig. 7 is a graph showing a relation between the environmental temperature and the light insertion loss or the light dividing ratio in the comparative example; Fig. 8 is a graph illustrating a relation between the environmental temperature and the light insertion loss or the light dividing ratio in the embodiment according to the invention; Fig. 9 is a graph depicting a relation between applied voltage and time; Figs. 10a, 10b and 10c are graphs respectively showing a relation between light intensity and time in the optical waveguide device according to the invention; Fig. 11 is a plan view illustrating another embodiment of the optical waveguide device according to the invention; Fig. 12 is a graph depicting a relation between the environmental temperature and the light insertion loss or the light dividing ratio in the embodiment shown in Fig. 11; and Fig. 13 is a plan view of an apparatus for measuring a light insertion loss and a light dividing ratio of an optical waveguide device. In the invention, the electrical conduction between the first conductive layer and the second conductive layer is preferably performed in one of the following methods. (1) When control electrodes are formed on a surface (optical waveguide forming side) of the optical waveguide substrate, conduction patterns for the electrical conduction are formed thereon at the same time. According to this method, the conduction patterns can be formed at the same time when the control electrodes are formed by the screen printing method or the vapor deposition method, and thus the electrical conduction can be performed simply. However, in this method, the conduction patterns become sometimes a cause of the light insertion loss of the optical waveguide. (2) On the surface (optical waveguide forming side) of the optical waveguide substrate, the first conductive layer and the second conductive layer are connected by means of wires. In this case, the wires are passed above the optical waveguide. Therefore, since the wires are not electrically conducted to the optical waveguide, there is no possibility to generate the light insertion loss in the optical waveguide. However, it is necessary to electrically conduct the wires to respective conductive layers. (3) The first conductive layer and the second conductive layer are respectively formed as a part of the same integrated conductive layer. Also in this case, the integrated conductive layer can be formed at the same time when the control electrodes are formed by the screen printing method or the vapor deposition method. Especially, in the active optical waveguide device having a construction such that, the control electrodes are preferably arranged at outer portions and inner portions of respective divided optical waveguides, and control voltages can be applied to respective divided optical waveguides by the control electrodes, the first conductive layer and the second conductive layer can be electrically conducted to the control electrodes of the outer portions respectively. Moreover, in the active optical waveguide device. mentioned above, the first conductive layer and the second conductive layer can be electrically isolated from the control electrodes of the outer portions respectively. In this case, since potentials at the first conductive layer and the second conductive layer are not affected by a potential applied to the control electrode, the pyroelectrical effects can be preferably eliminated. Further, in the functional portions mentioned above, third conductive layers are formed on regions between a plurality of divided optical waveguides, and the third conductive layers are electrically conducted to at least one of the first conductive layer and the second conductive layer. In this case, even in an inner region of the functional portions, the pyroelectrical effects can be preferably eliminated. Moreover, in the active optical waveguide device having a construction such that, the control electrodes are arranged at outer portions and inner portions of respective divided optical waveguides, and control voltages can be applied to respective divided optical waveguides by the control electrodes, it is preferred to isolate the third conductive layers from the control electrodes of the inner portions of respective divided optical waveguides. If the third conductive layers are electrically conducted to the control electrodes of the inner portion, potentials of respective conductive layers are affected by potential variations of the control electrodes of the inner portions. Especially, in the case that the first conductive layer and the second conductive layer are electrically conducted to the control electrodes of the outer portions, it is necessary to electrically isolate the third conductive layer from the control electrodes of the inner portions. As for ferroelectric crystals, it is preferred to use one or more ferroelectric crystals selected from the group consisting of LiNbO 3 , LiTaO 3 , Li(Nb x , Ta 1-x ), BaTiO 3 , ZnO, NH 4 H 2 PO 4 , KH 2 PO 4 and KTiOPO 4 . Moreover, it is more preferred to use one or more ferroelectric crystal selected from LiNbO 3 , LiTaO 3 and Li(Nb x , Ta 1-x ). As for the optical waveguide, it is preferred to use (1) optical waveguide formed by thermally diffusing titanium or copper into ferroelectric crystal, or (2) optical waveguide formed by proton-exchange process in which lithium atom in LiNbO 3 crystal, LiTaO 3 crystal or Li(Nb x , Ta 1-x ) O 3 crystal is exchanged by hydrogen atom. In addition, the present invention can be applied to optical waveguides of ridge type, optical waveguides of dielectric load type, optical waveguide of metal load type, etc. In the optical waveguide devices according to the invention, at least one functional portions for dividing or coupling lights is arranged in the optical waveguide. A shape of the optical waveguide may be planar or may be curvature. The functional portions may be so-called Y-branch portion or directional couplers. The Y-branch portion is formed by coupling one row of the optical waveguide and two rows of the optical waveguides, and functions to divide and/or couple lights. The directional coupler is formed by arranging adjacent optical waveguides very closely and functions to shift a guided light from on optical waveguide to the other optical waveguide. Therefore, a guided light can be divided by the directional coupler at any light dividing ratios. Moreover, the directional coupler can be used together with the Y-branch. The optical waveguide devices according to the invention can be used for active devices and passive devices. As for the passive devices, use is made of beam splitters, star couplers having a waveguide of cascade type consisting of a plurality of Y-branch portions, and so on. As for the active devices, use is made of light intensity modulators, phase modulators, optical switching devices, acoustic-optics devices, logic devices for optical computer (AND element, OR element), multiple modulators having such a construction that optical modulation electrodes are arranged to the star coupler, and so on. Especially in the optical modulators of the fiber optic gyroscope among the active devices, if the light dividing ratio of the divided lights ins out of the predetermined value, the rotation rate of the optical system to be measured is extraordinarily varied, and thus it is not possible to perform the precise measurement. Therefore, the optical waveguide devices according to the invention can be preferably used for the above-mentioned aims. Moreover, in the Mach-Zehnder light intensity modulator, there is a problem such that the extinction ration is varied. The present invention can preferably eliminate such a problem. Further, in the optical waveguide devices for dividing lights such as the beam splitters, the optical switching devices, the logic devices, and so on, they output erroneous values if the divided lights are varied. Therefore, the second aspect of the invention can be preferably applied for such devices. Especially, in the optical switching devices and the logic devices, ON-OFF states are detected from the divided lights transmitted through two rows of optical waveguides. Therefore, if the divided lights are varied over the threshold level, the ON state and the OFF state are exchanged, and thus it is not possible to use them. Further, in the star coupler and the multiple modulators, a plurality of Y-branch portions are arranged to make the cascade construction. Therefore, if the light dividing ratio of respective Y-branch portions is varied out of the predetermined value, these variations are multiplied from input to output. Therefore, even if the variation of the light dividing ratio at one Y-branch portion is for example 5%, the multiplied variations become extraordinarily large value. The invention can be preferably applied to star couplers and multiple modulators. As for the formation of the first, second and third conductive layers according to the invention, use is made of the following methods (1)∼(4): (1) By means of the vapor deposition method, a thin layer made of semiconductors or metals such as indium oxide, indium oxide-tin oxide, copper oxide, Si/Ge, and so on is formed. (2) On the crystal plane, a paste layer including semiconductors or metals such as indium oxide, indium oxide-tin oxide, copper oxide, Si/Ge, and so on is arranged, and the arranged paste layer is fired. (3) By performing a plasma treatment or an oxidizing treatment with respect to the crystal plane, a crystal structure of the crystal plane is deteriorated to form the conductive layer. (4) A thin layer made of semiconductors or metals having conductive properties and light shielding properties such as carbon black, graphite, and so on is formed. The thin layer is formed by arranging a paste layer including carbon black or carbon on the crystal plane and firing the paste layer. Where two conductive layers are connected electrically, if a resistivity of the conducted portion is large, it is not possible to neutralize the charges generated in the first crystal plane quickly. Therefore, a resistivity R of the conducted portion should be set in such a manner that electric fields applied to the optical waveguide portion is always below 10 V/cm under thinkable temperature variation rate. Under various temperature condition, a voltage V applied to the optical waveguide portion due to the pyroelectrical effects is obtained from the following formula. V = QR/d where, Q is the charges (coulomb) generated during a unit time, which depends on the environmental temperatures, R is a resistivity (Ω) of the conducted portion and d is a distance between the first crystal plane and the second crystal plane. Accordingly, in order to limit the voltage V applied to the optical waveguide portion under thinkable temperature variation rate below 10 V/cm, 10>QR/d should be satisfied. That is to say, the resistivity R of the conducted portion should be designed to satisfy R<10d/Q. Moreover, if it is necessary that the optical waveguide device performs always in a good condition under thinkable environmental temperature, the resistivities of both of the first conductive layer and the second conductive layer may be set below 100 Ω/cm 2 . Hereinafter, the invention will be explained, with reference to the drawings. Fig. 1 is a plane view showing one embodiment of the invention. In this embodiment, an optical waveguide device 51 is formed by an X-cut plate made of LiNbO 3 , LiTaO 3 or Li(Nb x , Ta 1-x ). A three dimensional optical waveguide 54 is formed in a major plane 51a of the optical waveguide device 51. The optical waveguide 54 comprises a linear portion 54a in a light input side, a Y-branch portion 54b and a pair of linear portions 54c. An optical fiber support substrate 53A is connected to alight input side of the optical waveguide substrate 51 to support an optical fiber 52A. The optical fiber 52A is optically coupled with the linear portion 54a. An optical fiber support substrate 53B is connected to a light output side of the optical waveguide substrate 51 to support a pair of optical fibers 52B. The optical fibers 52B are optically coupled with the linear portions 54c respectively. In this embodiment, a spontaneous electric polarization direction of the optical waveguide substrate 1 made of ferroelectric crystals is a direction shown by an arrow A and is substantially perpendicular to a light proceeding direction. It is sufficient to cross this spontaneous electric polarization direction to the light proceeding direction by a predetermined angle, but it is preferred to set the cross angle at 90°. The optical waveguide substrate 51 has a first crystal plane 51b and a second crystal plane 51c. Positive charges and negative charges are generated respectively in the first crystal plane 51b and the second crystal plane 51c due to the pyroelectrical effects. A first conductive layer 56a is formed in one region 55A extending from a boundary between the first crystal plane 51b and the major plane 51a to the Y-branch portion 54b, and a second conductive layer 56c is formed in the other region 55B extending from a boundary between the second crystal plane 51b and the major plane 51a to the Y-branch portion 54b. In this embodiment, the first conductive layer 56a and the second conductive layer 56c are integrated to construct an integrated conductive layer 56. Next, a phase of the integrated conductive layer 56 will be explained. Respective conductive layers 56a, 56c has a rectangular shape. Outer sides of respective conductive layers 56a, 56c are linear along a boundary between the major plane 51a and respective crystal planes. Inner sides of respective conductive layers 56a, 56c are substantially along a shape of the optical waveguide. Respective conductive layers 56a, 56c are integrated by a connection portion 56e, and the connection portion 56e is arranged on the optical waveguide 54. In Fig. 1, respective conductive layers 56a, 56c are arranged symmetrically with respect to a horizontal line. Thin control electrodes 56b, 56d are extended parallelly from respective conductive layers 56a, 56c. Respective linear portions 54c is arranged between inner control electrode 57 and respective control electrodes 56b or 56d. Figs. 2 and 3 are plane views respectively showing another embodiment according to the invention. In Figs. 2 and 3, portions similar to those of Fig. 1 are denoted by the same numerals, and the explanations thereof are omitted here. In the optical waveguide device shown in Fig. 2, a pair of conductive layers 66A and 66B are arranged outside of the optical waveguide 54 and are arranged symmetrically with respect to a horizontal line. The conductive layer 66A comprises a first conductive layer 66a and a control electrode 66b. The conductive layer 66b comprises a second conductive layer 66c and a control electrode 66d. Respective conductive layers 66a, 66c has a rectangular shape. Outer sides of respective conductive layers 66a, 66c are linear along a boundary between the major plane 51a and respective crystal planes. Inner sides of respective conductive layers 66a, 66c are substantially along a shape of the optical waveguide. Thin control electrodes 66b, 66d are extended parallelly from respective conductive layers 66a, 66c. Respective linear portions 54c is arranged between the inner control electrode 57 and respective control electrodes 56b or 56d. Moreover, the first conductive layer 66a and the second conductive layer 66c are electrically conducted by a connection member 60. As for the connection member 60, use is made of a conductive stripe, a conductive wire and so on. In Fig. 3, a conductive layer 76 is isolated from a pair of control electrodes 58. Therefore, the conductive layer 76 has no voltage apply function. Respective linear portions 54c are arranged respectively between the control electrode 57 and one of the control electrodes 58. The conductive layer 76 comprises a first conductive layer 76a, a second conductive layer 26b and a connection portion 76c. Respective conductive layers 76a, 76b has a rectangular shape. Outer sides of respective conductive layers 76a, 76b are linear along a boundary between the major plane 51a and respective crystal planes. Inner sides of respective conductive layers 76a, 76b are substantially along a shape of the optical waveguide. The connection portion 76c is arranged on the optical waveguide 54. In the embodiments shown in Figs. 1 to 3, the first conductive layer and the second conductive layer have a rectangular shape, and the inner sides of the conductive layers are along a shape of the optical waveguide. Therefore, a space between respective conductive layers and the optical waveguide and a space between the major plane 51a and an edge portion can be small, and thus the pyroelectrical effects for the Y-branch portions can be minimized. Figs. 4 and 5 are plane views respectively showing one embodiment of Mach-Zehnder light intensity modulator according to the invention. In the embodiments shown in Figs. 4 and 5, an optical waveguide 64 comprises a linear portion 64a in a light input side, a Y-branch portion 64b, a pair of linear portion 64c, a Y-branch portion 64d and a linear portion 64e in a light output side. The optical fiber 52A is coupled to the linear portion 64a, and the optical fiber 52B is coupled to the linear portion 64e. In the embodiment shown in Fig. 4, the conductive layer 56 is formed only in the side of the Y-branch portion 64b. Respective conductive layers 56a, 56c has a rectangular shape. Outer sides of the conductive layers 56a, 56c are linear along a boundary between the major plane 51a and respective crystal planes. Inner sides of the conductive layers 56a, 56c are substantially along a shape of the optical waveguide. Respective conductive layers 56a, 56c are integrated at the connection portion 56e, and the connection portion 56e is arranged on the optical waveguide 14. In Fig. 4, respective conductive layers 56a, 56c are arranged symmetrically with respect to a horizontal line. Thin control electrodes 56b, 56d are extended parallelly from respective conductive layers 56a, 56c. A control electrode 67 arranged between a pair of the linear portions 64c comprises a rectangular portion 67a and a triangular portion 67b. Respective linear portions 64c is arranged between the rectangular portion 67a and the control electrodes 56b or 56d. The triangular portion 67b is extended toward the Y-branch portion 64b. In the embodiment shown in Fig. 4, almost all region 55A extended from a boundary between the first crystal plane 51b and the major plane 51a to the Y-branch portion 64b is covered with the first conductive layer 56a. Moreover, almost all region 55B extended from a boundary between the second crystal plane 51c and the major plane 51a to the Y-branch portion 64b is covered with the second conductive layer 56c. In the embodiment shown in Fig. 5, almost all region, extend from a hexagonal region formed by the Y-branch portions 64b, 64d and a pair of linear portions 64c to edge portions of the major plane 51a, is covered with a conductive layer 86. Moreover, almost all inner portion of the hexagonal region is covered with a control electrode 77. The conductive layer 86 functions to act as an opposed electrode of the control electrode 77. Further, a shape of the conductive layer 86 will be explained. In this embodiment, almost all region 55A is covered with a first conductive layer 86a and almost all region 55B is covered with a second conductive layer 86c. In a side of the Y-branch portion 64d, almost all region 55C extended from a boundary between the first conductive layer 51b and the major plane 51a to the Y-branch portion 64d is covered with a first conductive layer 86f. Moreover, almost all region 55D extended from a boundary between the second crystal plane 51c and the major plane 51a to the Y-branch portion 64d is covered with a second conductive layer 86g. Respective conductive layers 86a, 86c, 86f and 86g has a rectangular shape. Outer sides of respective conductive layers 86a, 86c, 86f and 86g are linear along a boundary between the major plane 51a and respective crystal planes. Inner sides of these conductive layer are linear along a shape of the optical waveguide. The conductive layers 86a and 86c are integrated at a connection portion 86e, and the connection portion 86e is arranged on the optical waveguide 64. The conductive layers 86f and 86g are integrated at a connection portion 86h, and the connection portion 86h is arranged on the optical waveguide 64. In Fig. 5, the conductive layers 86a and 86c and the conductive layers 86f and 86g are respectively arranged symmetrically with respect to a horizontal line. The conductive layers 86a and 86f and the conductive layers 86c and 86g are respectively connected by thin control electrodes 36a and 36d. The control electrode 77 comprises a rectangular portion 77a and triangular portions 77b and 77c. The linear portions 64c are respectively arranged between the rectangular portion 77a and the control electrode 86b or between the rectangular portion 77a and the control electrode 86d. The triangular portion 77b is extended toward the Y-branch portion 64b, and the triangular portion 77c is extended toward the Y-branch portion 64d. Hereinafter, actual examples according to the invention will be explained. Example 1 The optical waveguide devices according to the examples of the invention and according to the comparative examples were manufactured. Then, properties as the phase modulator for use in fiber optics gyroscope were measured. As for the optical waveguide devices according to the examples of the invention, use was made of the optical waveguide device having the construction shown in Fig. 1. The optical waveguide devices were manufactured in the following manner. At first, a disk-shape wafer made of LiNbO 3 having X-cut plane was prepared. The wafer head a diameter of 3 inches and a thickness of 1 mm. A titanium thin layer having a width of 3 µ m and a thickness of 500 angstrom was formed on the wafer by using the photolithography technic, and a titanium component was diffused into the LiNbO 3 wafer by effecting a heat treatment such as 1000°C×6 hours to form the titanium diffused optical waveguide 54. After that, conductive layer 56 and the control electrode 57, both made of metal layers, were respectively formed by a spraying method and so on. Gap distances between the electrode 57 and the control electrodes 56b or 56d were 10 µ m, and a length of the electrode 57 was 15 mm. From this wafer, the optical waveguide substrates 51 for one chip were cut out. To the both ends of the optical waveguide substrate 51, the optical fibers 52A and 52B were connected as shown in Fig. 1 to obtain specimens according to the invention. At the same time, the optical waveguide devices having a construction shown in Fig. 6a according to the comparative example were manufactured in the same manner as mentioned above. However, the specimens according to the comparative examples have electrodes 58 in spite of the conductive layer 56. Then, the optical waveguide devices according to the examples of the invention and according to the comparative examples were accommodated in the temperature adjusting apparatus shown in Fig. 13. Then, a light having a wavelength of 0.85 µ m was introduced from a light source, and output lights transmitted through a pair of optical fibers 52B were measured by a pair of light amount measuring apparatuses. After that, a light insertion loss and a light dividing ratio were calculated from respective measured values. During this measuring operation, as shown in Figs. 7 and 8, the environmental temperature of the optical waveguide devices were varied. That is to say, the environmental temperature was varied stepwise from 20°C to -40°C. In this case, a holding time at respective temperatures was about 45 minutes, and an ascending rate and a descending rate of temperature was 80°C/hour. As a result, Fig. 7 shows the results of the comparative example, and Fig. 8 shows the results of the example according to the invention. As shown in Fig. 7, in the comparative example, when the environmental temperature ascended or descended, both of the light insertion loss and the light dividing ratio were varied largely. It should be noted that, in Fig. 7, a region of the light insertion loss more 4dB and a region of the light dividing ratio more than 30:70 were cut out. Contrary to this, in a comporative example both of the light insertion loss and the light dividing ratio were not varied as shown in Fig. 8. In this case, a variation of the light insertion loss was less than 0.4dB, and a variation of the light dividing ratio was in a range of 50:50∼52:48. Moreover, the optical waveguide devices according to the present invention and according to the comparative example were assembled in the fiber optics gyroscopes as modulators. Then, as shown in Figs. 7 and 8, the environmental temperature was varied. As a result, in the comparative example, the calculated rotation angular rate was varied by more than 20%, and sometimes it was not possible to measure the rotation angular rate. Further, the optical waveguide devices shown in Figs. 2 and 3 were manufactured in the same manner mentioned above. Then, the same experiment was performed with respect to respective optical waveguide devices. As a result, the same results as those shown in Fig. 8 were obtained. Example 2 The optical waveguide devices according to the examples of the invention and according to the comparative examples were manufactured. Then, properties of the Mack-Zehnder light intensity modulator were measured. As for the optical waveguide devices according to the examples of the invention, use was made of the optical waveguide devices having the constructions shown in Figs. 4 and 5. The optical waveguide devices were manufactured in the following manner. At first, a disk-shape wafer made of LiNbO 3 having X-cut plane was prepared. The wafer had a diameter of 3 inches and a thickness of 1 mm. A titanium thin layer having a width of 3 µ m and a thickness of 500 angstrom was formed on the wafer by using the photolithography technic, and titanium component was diffused into the LiNbO 3 wafer by effecting a heat treatment such as 1000°C×6 hours to form the titanium diffused optical waveguide 64. After that, the conductive layers 56, 86 and the control electrodes 67, 77, both made of metal layers, were respectively formed by a spraying method and so on. Thicknesses of the conductive layers 56, 86 and the control electrodes 67, 77 were 2500 angstrom. From this wafer the optical waveguide substrates 51 for one chip were cut out. To the both ends of the optical waveguide substrate 51, the optical fibers 52A and 52B were connected as shown in Figs. 4 and 5 to obtain specimens according to the invention. At the same time, the optical waveguide devices having a construction shown in Fig. 6b according to the comparative example were manufactured in the same manner as mentioned above. However, the specimens according to the comparative example have electrodes 18 and 37 in spite of the conductive layers 56, 86 and the control electrodes 67, 77. Then, the optical waveguide devices according to the examples of the invention and according to the comparative examples were accommodated in the temperature adjusting apparatus shown in Fig. 13. Then, a light having a wavelength of 0.85 µ m was introduced from a light source, and output lights transmitted through a pair of optical fibers 52B were measured by a pair of light amount adjusting apparatuses. After that, a light insertion loss and a light extinction ratio in a temperature stable state (-20°C) were measured. Moreover, the light insertion loss and the light extinction ratio were measured when a temperature was varied at a rate of 80°C/hour. In the measurement of the light extinction ratio, an applied voltage was varied reciprocally as shown in Fig. 9. In Fig. 9, Vπ means 5 V. As a result, in the specimen according to the comparative example shown in 21b, a light intensity was varied as shown in Fig. 10a in response to the temperature variation. In this case, a light intensity had an arbitrary unit. This light extinction ratio was 0.6dB. Moreover, the light insertion loss in a temperature variable state was increased by 3dB as compared with that in a temperature stable state. In the embodiment shown in Fig. 4, the light intensity in a temperature variable state was varied as shown in Fig. 10b. This light extinction ratio was 2dB. Moreover, the light insertion loss in a temperature variable state was increased by 3dB as compared with that in a temperature stable state. In this manner, if the conductive layers 56a, 56c were arranged around the Y-branch portion 64b, the light extinction ratio in a temperature variable state can be extraordinarily improved. In the embodiment shown in Fig. 5, the light intensity in a temperature variable state was varied as shown in Fig. 10c. This light extinction ratio was 20dB. Moreover, the light insertion loss in a temperature variable state was increased by only less than 0.1dB as compared with that in a temperature stable state. In this manner, if the conductive layers 86a, 86c, 86f, 86g were arranged around the Y-branch portions 64b and 64d, both of the light extinction ratio and the light insertion loss in a temperature variable state can be extraordinarily improved. Fig. 11 is a plan view showing another embodiment of the optical waveguide device according to the invention, which relates to the optical waveguide substrate having Y-branch portions shown in Figs. 1 to 3. In the embodiment shown in Fig. 11, a conductive layer 76 is isolated from a pair of the control electrodes 58, and does not function to apply a voltage. The linear portions 54c in a light output side are arranged respectively between the control electrode 57 and the control electrode 58. The integrated conductive layer 76 comprises a first conductive layer 76a, a second conductive layer 76b and a connection portion 76c. The conductive layers 76a, 76b have a rectangular shape. Outer sides of the conductive layers 76a, 76b are linear along a boundary between the major plane 51a and respective crystal planes. Inner sides of the conductive layers 76a, 76b are along a shape of the optical waveguide. The connection portion 76c is arranged on the optical waveguide 54. In an inner portion of the Y-branch portion 54b, a third conductive layer 80 having a triangular shape is formed, and the third conductive layer 80 and the second conductive layer 76b are electrically conducted by means of the connection member 60. The third conductive layer 80 and the control electrode 57 are isolated. Example 3 The optical waveguide device show in Fig. 11 was manufactured, and the same experiments as those of the example 1 were performed. As is the same as the example 1, the titanium diffused optical waveguide 54 was formed in the optical waveguide substrate 51 by using a titanium diffusing method. After that, the conductive layers 76, 80 and the control electrodes 57, 58, both made of metal layers, were formed by a spraying method and so on. Thicknesses of the conductive layers 76, 80 and the control electrodes 57, 58 were 2500 angstrom. A gap distance between the control electrodes 57 and 58 was 10 µ m, and a length of respective control electrodes was 15 mm. From this wafer, the optical waveguide substrates 51 for one chip were cut out. To both ends of the optical waveguide substrate 51, the optical fibers 52A, 52B are connected as shown in Fig. 11 to obtain the specimens. The thus obtained optical waveguide device shown in Fig. 11 was accommodated in the temperature adjusting apparatus. Then, a light having a wavelength of 0.85 µ m was introduced from the light source, and output lights transmitted through a pair of optical fibers 2B were measured by a pair of light amount measuring apparatuses. After that, a light insertion loss and a light dividing ratio were calculated from respective measured values. During this measuring operation, as shown in Fig. 12, the environmental temperature of the optical waveguide devices were varied. That is to say, the environmental temperature was varied stepwise from 80°C to -40°C. In this case, a holding time at respective temperature was about 45 minutes, and an ascending rate and a descending rate of temperature was 80°C/hour. As a result, as shown in Fig. 12, both of the light insertion loss and the light dividing ratio were not varied. That is to say, in a wide temperature range of -40°C ∼ +80°C, a variation of the light insertion loss was 0.3dB and a variation of the light dividing ratio was in a range of 48:52 ∼ 49.8:50.2. Especially, in a temperature range of -40°C ∼ +20°C, a variation of the light insertion loss was less than 0.2dB, and a variation of the light dividing ratio was in a range of 48:52 ∼ 49:51. As mentioned above, according to the invention, it is possible to obtain the optical waveguide device showing no variations of the light insertion loss, the light dividing ratio and the light extinction ratio at functional portions in the optical waveguide even if the environmental temperature is varied.

Claims: An optical waveguide device comprising an optical waveguide substrate (51), an optical waveguide (54) formed in a major plane (51a) of said optical waveguide substrate (51), the optical waveguide comprising a linear portion (54a), and at least one functional portion (54b) for dividing or coupling light beams, including branch arms diverging from a junction point, said waveguide substrate (51) being made of a ferroelectric crystal whose spontaneous electric polarization direction is transverse to the normal of the major plane (51a) and to the linear portion (54a), a first crystal plane (51b) formed in said optical waveguide substrate in which charges of one polarity are generated due to pyroelectrical effect, a second crystal plane (51c) formed in said optical waveguide substrate in which charges of the other polarity are generated due to pyroelectrical effect, a first conductive layer (56a) formed in one region (55A) defined from a boundary between said first crystal plane (51b) and said major plane (51a) to said functional portion (54b), and a second conductive layer (56c) formed in another region (55B) defined from a boundary between said second crystal plane (51b) and said major plane (51a) to said functional portion (54b), wherein said first conductive layer (56a) and said second conductive layer (56c) are electrically connected, characterised in that inner edges of respective conductive layers (56a,56c) are disposed alongside said branch arms of the optical waveguide (54). An optical waveguide device according to claim 1, wherein said first conductive layer (56c) and said second conductive layer (56c) are electrically connected by wires (60) or conductive strips. An optical waveguide device according to claim 1, wherein said first conductive layer (56a) and said second conductive layer (56c) are formed integrally. An optical waveguide device according to any one of claims 1 to 3, wherein control electrodes (56b, 56d,57) are formed on said optical waveguide substrate (51) at an outer portion and an inner portion of said functional portion of said optical waveguide (54) so as to apply a voltage to said optical waveguide (54) and said first conductive layer (56a) and said second conductive layer (56c) are electrically connected to said respective control electrodes (56b,56d) at an outer portion. An optical waveguide device according to any one of claims 1 to 3, wherein control electrodes (58,57) are formed on said optical waveguide substrate (51) at an outer portion and an inner portion of said functional portion of said optical waveguide (54) so as to apply a voltage to said optical waveguide (54), and said first conductive layer (76a) and said second conductive layer (76b) are electrically isolated from said control electrodes (58) at said outer portion. An optical waveguide device according to any one of claims 1 to 5, wherein a third conductive layer (80) is formed on said optical waveguide substrate (51) in a region between divided optical waveguides (54c), and said third conductive layer (80) is electrically connected at least one of said first conductive layer (76a) and said second conductive layer (76b). An optical waveguide device according to claim 6, wherein control electrodes (58,57) are formed on said optical waveguide substrate at an outer portion and an inner portion of said functional portion of said optical waveguides so as to apply a voltage to said optical waveguide (54) and said third conductive layer (80) is electrically isolated from said control electrode (57) at said inner portion. An optical waveguide device according to any one of claims 1 to 7, wherein said functional portion (54b) is a Y-branch portion or a directional coupler.

Inventors: ASANO MASATO, EJIRI TETSUYA, ASANO, MASATO, EJIRI, TETSUYA
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1994
Main classification: G02F1
IPC codes: G02F1